Mesorhaga falciunguis n. sp. (Figs 2 b-c)

Type material. Holotype ɗ, paratype Ψ, Mexico: MORELOS: 34 miles south of Cuernavaca, 2600 ft, 4.viii.1954, J.G. Chillcott (CNC); paratypes: Ψ, 12 miles east of Cuernavaca, 4400 ft, 14.viii.1954, J.G. Chillcott (CNC); 2 ɗ, Cuernavaca, 12–19.vii.1961, R.R. Driesbach (CAS); ɗ, Ψ, 6 miles north of Cuernavaca, 8000 ft., 15.viii.1954, R.R. Driesbach (CAS); 6 ɗ, 2 Ψ, Cuernavaca, 1.viii.1938, L.J. Lipovsky (CAS).

Additional material. Mexico: DISTRITO FEDERAL: ɗ, Mexico City, 8000 ft., 16.viii.1954, R.R. Driesbach (CAS). MORELOS: ɗ, Yecapixtla, 11.vii.1970, J. & M. Sedlacek (BPBM); possible Ψ, Amecameca, 8000 ft, 17.viii.1956, R.R. Driesbach (CAS).

Description. Male: body length: 3.8–3.9 mm; wing: 3.3 x 1.3 mm.

Head: 2 strong postvertical setae present in line with the postoculars; weak proclinate vertical present on lateral frons; vertex, frons, face, and clypeus metallic blue-green, with some silvery pruinosity; clypeus slightly narrowed distally; palp dark brown, with apical seta; proboscis brown; antenna black; pedicel with long ventral seta; 1st flagellomere subrectangular; arista dorsal; l postcranium ventrally with pale setae.

Thorax: metallic green with bronze reflections; pleura with some grey pruinosity; ac absent, present as single pair long setae anteriorly, or as single pair of anterior setae and 2–3 unpaired setae posteriad; lateral scutellar setae about quarter length of medians.

Legs: all coxae, trochanters, FI and FII almost to apices, and all leg III dark brown; femoral apices, tibiae and basitarsi I and II yellowish; distal tarsomeres I and II dark brown; CI and CII with pale anterior hairs and black anterolateral setae; CIII with black lateral seta and some shorter whitish setae; all tibiae bare of major setae, but covered with short, almost erect setae. I: 3.6; 3.4; 3.0/ 0.8/ 0.4/ 0.2/ 0.3; FI with pale ventral setae on basal three-quarters; It5 with posterior claw (Fig. 2 c) greatly enlarged and curved (MSSC). II: 4.6; 5.5; 3.5/ 1.5/ 1.0/ 0.7/ 0.5; FII with white av and pv setae. III: 5.5; 6.5; 2.7/ 2.1/ 1.3/ 0.7/ 0.5; FIII with white av and pv setae.

Wing: with faint brown wash over wing anteriad of vein R4+5; M with right-angle curve at 1/3 between dm-cu crossvein and wing apex; CuAx ratio: 0.8; lower calypter yellow, with black rim and fan of black setae; halter black.

Abdomen: terga 1–4 metallic blue-green with bronze reflections, covered with short dark setae dorsally and pale setae laterally; tergum 1 with posterolateral black setae; terga 5–6 black, with bronze reflections, expanded and rounded ventrally with abundant black setae; hypopygium (Fig. 2 b) dark brown; phallus and hypandrium elongate and narrow; epandrial lobe ovate, with setae at 3/4 and apex, strong seta basally; surstylus with 2 arms and setae as figured; cercus slightly curved and tapering, apically clavate.

Female: similar to male except: 4 postvertical setae present; vertical seta strong; ac also with single pair of anterior setae and 4 unpaired almost uniseriate setae posteriad; femora ventrally bare; tibiae also bare of major setae and covered with short, almost erect setae; claws of leg I unmodified; halter yellow.

Remarks. Mesorhaga falciunguis occurs above 800 m in the Central Plateau of Mexico, from the Federal District to Morelos. Males have a long, sickle-shaped posterior claw on leg I (Fig 2 c), hence the specific epithet from the Latin, falci for sickle, unguis for claw. Also of interest, neither sex has strong setae on the tibiae, unusual for the genus.

A badly damaged series of 3 males [Mexico: west slope of Popocatepetl, 2400 –2600 m, 10.vii.1970, J. & M. Sedlacek (BPBM)] is close to M. falciunguis, but the male posterior claw on leg I is short. Either these specimens represent a related undescribed species, or M. falciunguis shows intraspecific variation in the claw length of leg I.